Molecular beam experiments with specially prepared beams allow the study of the interaction of very reactive species at surfaces. In the present case the focus is primarily on the interaction of N-atoms with surfaces. In this chapter questions that will be addressed include: - What is the scattering pattern and energy transfer of N-atoms at surfaces? - Can adsorption of N-atoms lead to a passive layer that is not reactive to incident N-atoms? - Conversely, can N-atoms remove N-atoms in an Eley-Rideal or hot atom reaction? - Does the electronic state of the atoms matter? - Can the interaction already be described by state-of-time art theory? The methods used will be introduced with examples of fast Ar scattering from Ag(111). Subsequently, t...